Application of lithium-ion secondary batteries to electronic appliances such as cellular phones and notebook computers has been increasing for their high energy density. In a lithium-ion secondary battery, lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate and the like are used as a cathode active material, and graphite and the like are used as an anode active material. A lithium-ion secondary battery is typically composed of electrodes made of the active materials, a separator which is a porous sheet, and an electrolyte in which a lithium salt is dissolved. Such a lithium-ion secondary battery has a high battery capacity and output as well as a good charge-discharge property, and a service life thereof is relatively long.
Although a lithium-ion secondary battery has an advantage of high energy density, it is accompanied by problems associated with safety since it employs a non-aqueous electrolyte. For example, since it contains a non-aqueous electrolyte, a component of the non-aqueous electrolyte possibly decomposes along with heat generation, causing internal pressure to raise, which may lead to defects such as a swollen battery. Further, if a lithium-ion secondary battery is overcharged, defects such as heat generation possibly occur. Moreover, there is a risk that heat generation or other defects are also caused by occurrence of an internal short-circuit. Heat generation of a battery sometimes leads to ignition, and thus, safety measures directed to suppressing it are important.
Examples of means for enhancing safety of battery include prevention of elevation of internal pressure by means of a safety valve, and current interruption at the time of heat generation by incorporating a PTC (Positive Temperature Coefficient) element, whose resistance increases as temperature increases. For example, a method is known in which a PTC element is furnished to a cap portion of the cathode of a cylindrical battery.
However, the method of furnishing a PTC element to the cap portion of the cathode is accompanied by a problem that it is not possible to prevent heat generation due to an internal short-circuit, overcharging or the like.
A separator incorporated in a lithium-ion secondary battery has a function that, when abnormal heat generation occurs, a resin melts and occludes pores of the separator, lowering ion-conductivity so as to suppress increase of short-circuit current. However, a separator located distant from the heat-generating portion does not always melt, and when heat is generated so much that the heat distortion temperature of the resin is exceeded, the separator shrinks with heat, which brings about a risk that a short-circuit is caused adversely. As discussed above, the means for preventing heat generation due to an internal short-circuit, overcharging or the like still have room for improvement.
To resolve the problem of an internal short-circuit, a cathode is proposed which has a PTC layer composed of a crystalline resin and conductive particles. Such a PTC layer has a property that the resin expands near the melting point of the crystalline resin which causes breakage of the network of the conductive particles, and thus, resistance is increased greatly. In Patent Document 1, it is disclosed that carbon particles and a crystalline resin are heated and mixed, resulting mixture is processed into a sheet, and then annealed to give a PTC layer formed on a current collector. Further, in Patent Document 2, a PTC layer is disclosed which includes a crystalline resin such as polyethylene, a conductive material and a binder, and is 5 μm or less. In Patent Document 3, a PTC layer is disclosed which is composed of a polyethylene wax emulsion and carbon microparticles.